If you have a chance to see the Fresnel lens, you will see thinner and thicker features on the strip or ball. If an object moves across the field of view, its temperature is alternately passing to the PIR sensor or not (diagrams in the Specifications). This enables the sensor to detect moving warm objects. The same thing you can make with a grid, thin strips cut from Teflon with similar gaps of air. Using Teflon (or another dielectric) modulates the response like Fresnel lens does. If you use wire grid, metal strips may radiate the high ambient temperature and overload the sensor.